The anti hormonal drug tamoxifen is approved for the treatment of women with early and advanced and early stage estrogen receptor (ER)-positive breast cancer. Tamoxifen is known to exert its effects on breast cancer cells by binding to the estrogen receptor, thereby preventing the binding of estrogen. However, a major clinical problem is that a large number of patients with ER-positive breast tumors either do not respond to tamoxifen therapy or develop resistance to it. Although several plausible reasons for such resistance have been suggested, the mechanisms of resistance to tamoxifen therapy remain largely unclear. ER mediates effects of estrogen in promoting proliferation of breast cancer cells. Tumor suppressor protein p53 guards against tumorigenesis by mediating cell cycle arrest and/or apoptosis in response to various cellular stressors. Whether there is a direct link between these two antagonistic pathways has remained unclear. Our experiments have demonstrated that ER1 binds to p53 and represses its function in human breast cancer cells. Importantly, estrogen enhances ER1-p53 interaction, whereas anti-hormonal drugs tamoxifen and fulvestrant disrupt the interaction. Results from our retrospective clinical studies correlate with our cellular and molecular observations and are consistent with the reported association of wild type p53 status with better response to tamoxifen therapy. Based on these findings, it is hypothesized that relieving suppression of wild type p53 by ER1 could be an important mechanism underlying tamoxifen action. The benefit of tamoxifen's effect becomes irrelevant if the tumor has mutant (and therefore dysfunctional) p53. To test these hypotheses, a pilot randomized clinical trial of 50 pre- menopausal women with newly diagnosed ER-positive breast cancer expressing wild type p53 is proposed. The Specific Aims are: (1) Investigate the status of ER-p53 interaction in patients treated with 20 mg of tamoxifen for four weeks prior to surgery as compared to the interaction in untreated patients, and (2) Confirm the wild type status of p53 and analyze the functional status of p53 pathway by monitoring expression of selected p53-target genes in tumors in patients who have or have not been treated with 20 mg tamoxifen for four weeks prior to surgery. These studies will be conducted prospectively in newly diagnosed breast cancer patients at the host institution using a combination of clinical, pathological, cellular, and molecular biology expertise. The status of ER and p53, their interaction, and its consequences in breast tumors will be analyzed with immunohistochemical, cellular, and molecular approaches. The proposed work has the potential to bring about a paradigm shift to the analysis of mechanisms underlying resistance to tamoxifen therapy. PUBLIC HEALTH RELEVANCE: This research addresses resistance to tamoxifen therapy from a novel angle by proposing a pilot clinical trial to analyze if tamoxifen can restore functional tumor suppressor p53 by disrupting ER-p53 interaction. Results from this study will help develop strategies to identify breast cancer patients who would most benefit from tamoxifen therapy, and avoid unnecessary exposure to this drug and its numerous side effects. Thus, results from the proposed study could impact the development of individualized therapy for breast cancer patients.